JP-A-No. 59-184 744 describes a method in which high-refractive index layers of ZrO.sub.2 and/oz TiO.sub.2 and Iow-refractive index layers of SiO.sub.2 and/or Al.sub.2 O.sub.3 are provided on a glass substrate by alternating vacuum deposition. A heat treatment above approximately 450.degree. C., preferably 650.degree. to 700.degree. C., produces diffused layers approximately 3 to 10 nm thick, in between the aforesaid layers which improve the resistance to wear.
An interference mirror having an alternating layer sequence of mixtures of TiO.sub.2 and HfO.sub.2, TiO.sub.2 and ThO.sub.2 or ThO.sub.2 and HfO.sub.2 on the one hand and SiO.sub.2 on the other hand is disclosed in SU-A-No. 306 520. This interference mirror has a higher resistance to radiation from optical quantum generators than mirrors comprised of an alternating sequence of layers of TiO.sub.2 and SiO.sub.2. The layers are from ethoxide or salt solutions produced by means of a dip immersion method, respectively, and are subjected to heat treatments at 400.degree. C. for the mixed oxide layers and 500.degree. C. for the SiO.sub.2 layers.
JP-A-No. 59-102 201 discloses optical interference coatings consisting of an alternating layer sequence of Ta.sub.2 O.sub.5 and/or TiO.sub.2 on the one hand and SiO.sub.2 on the other hand, some of these layers or all the layers optionally containing P.sub.2 O.sub.5. The layers are produced from corresponding metal-organic compounds while being heated to 200.degree. or 550.degree. C., respectively. Because of the fact that the top layer consists of Ta.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 +TiO.sub.2 a high resistance against salt water, high temperatures and high humidity is obtained.
All the layer sequences known from said documents have in common that the TiO.sub.2 -containing layers are not crystalline because of the comparatively low temperatures used during the heat treatment. DE-A-No. 334 962 discloses that non-crystalline TiO.sub.2 layers are obtained at 500.degree. C. while crystalline TiO.sub.2 is not produced until 600.degree. C. (anatase) or 900.degree. C. (rutile).
DE-A-No. 3 227 096 describes an optical interference filter for applications at over 500.degree. C., which is comprised of an alternating sequence of, for example 27 layers of Ta.sub.2 O.sub.5 and SiO.sub.2. The Ta.sub.2 O.sub.5 layer may optionally include a low percentage of a different refractory oxide, for example TiO.sub.2. A heat treatment at temperatures below 1100.degree. C. produces a visible light-transmissive infrared-reflecting filter which is converted into a filter which scatters visible light and reflects infrared light when the layer sequence is heated for several hours to at least 1100.degree. C. in air.
Optical interference filters are used in, for example, laser technology. Optical filters are also used with incoherent light sources such as qas discharge lamps and halogen lamps to increase the luminous efficacy of the lamps, as color filters or colour correction filters and also as reflectors. When used with lamps, technically the most difficult problem is the manufacture of more efficient heat reflectors for the near infrared wavelength ranqes (0.75 to approximately 3.5 .mu.m).
For the material for the bulbs of such halogen incandescent lamps, quartz glass is the most suitable material, which does not start to crystallize until at temperatures above 1100.degree. C. In special cases doped quartz qlass or hard qlasses can alternatively be used.
The choice of SiO.sub.2 as the low refractive index filter material is based on the fact that the optical efficacy of an interference filter increases with increasing refractive index difference between the high and low refractive index materials and that SiO.sub.2 has one of the lowest refractive indices, (n=1.45).
The choice of materials with high refractive indices for the construction of a filter is determined by the following criteria:
(a) it must have the highest possible refractive index; PA1 (b) it must have an adequate adhesion to amorphous SiO.sub.2 (a-SiO.sub.2); PA1 (c) it must have the lowest possible thermal coefficient of expansion. PA1 88-95 Mole. % TiO.sub.2 and 5-12 Mole. % ZrO.sub.2, PA1 88-95 Mole. % TiO.sub.2 and 5-12 Mole. % HfO.sub.2, PA1 TiO.sub.2.ZrO.sub.2, TiO.sub.2.HfO.sub.2, TiO.sub.2.Nb.sub.2 O.sub.5, PA1 TiO.sub.2.Ta.sub.2 O.sub.5 and Ta.sub.2 O.sub.5.2 TiO.sub.2,
Since a SiO.sub.2 has a linear thermal coefficient of expansion of only 0.5.times.10.sup.-6 K.sup.-1, too high an expansion coefficient of the high refractive index material induces high stresses which when the filter is subjected to heat, result in cracking and destruction. Experience has taught that these effects are increasingly more serious for filters with an increasing filter thickness or an increasing number of layers, respectively.
If possible, no phase transformations should occur in the temperature range of interest, which may have an upper limit of for example 900.degree. or 1100.degree. C. Recrystallisation usually leads to the formation of microcracks, which for optical filters, cause unwanted light dispersion.